8 Acknowledgments

This work has been supported since the initial version in part by the National Science Foundation, Grant
Numbers PHY 96-00049, 00-96522, and 03-53180, and by the National Aeronautics and Space
Administration, Grant Number NAG5-10186. We also gratefully acknowledge the support of the Centre
National de la Recherche Scientifique, and the hospitality of the Institut d’Astrophysique de Paris, where
this update was completed. Comments from referees were particularly helpful in improving this
update.